This invention relates to marshalling procedures in optical TDMA (time division multiple access) systems. In such a system a plurality of outstations communicate with a basestation on a TDMA basis while the basestation may communicate with its outstations on a broadcast basis. In such a system there is a fundamental requirement that the system should be able to measure, and then equalise, the propagation delays on each spur linking an outstation with the basestation. This measurement can be made by transmitting signals at normal power from each of the outstations to the basestation, and noting the time of receipt of those signals. However, in a practical system the uncertainty in this time occasioned by the different propagation delays means that a substantial period of potential transmission time in the upstream (outstation to basestation) direction needs to be reserved for the receipt of these signals. This method is referred to as "serial marshalling" since it can not be carried out simultaneously with the upstream transmission of data traffic.
An alternative marshalling method, the method with which the present invention is particularly concerned, involves an initial marshalling procedure which involves the use of transmissions at power levels too low to have significant affect upon normal data traffic, so that these initial marshalling transmissions can therefore be transmitted in parallel with that data traffic. This involves the outstation transmitting a pseudo-random bit sequence which is detected at the basestation by cross-correlation. Such an initial marshalling method is for instance described in U.S. Pat. No. 5,528,596.
The optical source of an outstation is provided by a directly modulated semiconductor laser chip and, because the electro-optic conversion efficiency of such a device varies with temperature and with the effects of ageing, it is conventional practice to regulate its optical power output with the aid of a feedback control loop, deriving a feedback control signal from the photocurrent produced by a monitor photodetector positioned to intercept a part of the laser's emission. Typically this photodetector is positioned to intercept light emitted from the back facet of the laser.
For some general applications of digitally data modulated semiconductor laser sources it is sufficient for its feedback loop to respond to the mean level of optical power output of the laser, and for there to be, in consequence, no need for the photodetector's response to be fast enough to resolve individual data bits. On the other hand, in TDMA applications a response fast enough to resolve these bits is typically a desideratum, the meeting of which places an upper limit upon the size of the photosensitive area of the photodetector due to capacitance effects.
When initial marshalling commences, the output power of the outstation's laser chip must, under the least favourable conditions, be low enough to avoid producing unacceptably high corruption of data being simultaneously transmitted to the basestation from any other outstation. Typically this transmission will be at a power level too low for the cross-correlation performed at the basestation to detect it. The power level of the transmission must then be incremented in steps until it is large enough to be detected. The size of those steps is regulated on the one hand by the need to make them small enough for a single increment not to raise the power level from undetectable to data-corrupting, and on the other hand to make them large enough to ensure that detection will occur within a reasonably short time interval from the commencement of the initial marshalling.
Implicit in the foregoing is the fact that regulation of the optical power output of the outstation's laser chip is required, not only during data transmission, but also during the initial marshalling process, and that at least in the initial stages of the initial marshalling process, the optical output power of the laser chip needs to be regulated to a power level very much lower than that employed during data transmission, typically commencing at a power level in the region of 40 dB below that of the data bits. This commencement power level is liable to be so low as to provide an inadequate photocurrent from a back-facet monitor diode that is small enough to have a response fast enough to resolve individual data bits during data traffic transmission.
One way of circumventing the problem of having too low a light level, during at least the initial stages of initial marshalling for detection by a monitor photodiode fast enough to resolve individual data bits, is to create a look-up table that relates power output to drive current from calibration measurements made on a test jig before the laser is ever brought into service. (Such a test jig would derive its calibration from measurements made with its own photodetector rather than the monitor photodiode of the laser itself). Under these circumstances, due allowance can be made for the effects of temperature, but importantly not for those of ageing.
The absence of a means for making allowance for the effects of ageing gives rise to a magnitude of uncertainty in optical power level output for which due allowance must be made in the devising of the incremental power steps in a manner that avoids the risk of data corruption, and this in turn has the effect of lengthening the time taken to complete initial marshalling.